In a conventional surface acoustic wave ring filter, an input electrical signal is initially converted into two surface acoustic waves by means of a first converter, and a plurality of reflectors are used for respectively transmitting the two surface acoustic waves along different transmission lines or paths. The two surface acoustic waves transmitted along different transmission lines are applied to an output second converter which converts the two surface acoustic waves into a single electrical signal. If the lengths of the transmission lines were exactly equal to each other, the waves would be added to each other vectorally by the second converter and thus the magnitude of the output electrical signal is substantially the same as that of the input electrical signal. However, it is very difficult to match the phases of the two surface acoustic waves with each other inasmuch as it is necessary to make the lengths of the transmission lines exactly equal to each other.
According to recent sophisticated photolithographic technique it is possible to achieve the precision as accurate as one micron in manufacturing microcircuits such as integrated circuits. However, as will be described hereinlater, the accuracy to this extent is still insufficient and therefore, a practical filter cannot be manufactured. The difference in the lengths of the transmission lines results in the production of frequency response ripple components in the output electrical signal, while the magnitude of the output electrical signal is undesirably attenuated.